1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming clear, high-quality visual images.
2. Description of the Related Arts
In recent years, in image formation using electrophotography, a technique in which particles with small diameters are used has been proposed to form visual images with the high quality equal to that of offset printing.
For example, according to Japanese Patent Application Laid-open (kokai) No. 2000-81722, it is described that;
by using a toner comprising coloring particles including at least bonding resin and coloring agent,
(1) the volume mean particle diameter of the coloring particle is 2.0xcx9c5.0 xcexcm, the coloring particles with diameters of 1.0 xcexcm or less are 20% of toner particles in number or less and the coloring particles with diameters exceeding 5.0 xcexcm are 10% of the toner particles in number and,
(2) the coloring agent is pigment particle, a full-color image can be obtained that is as high-quality as, or more high-quality than an image obtained by offset printing, has a very high performance in reproducibility of thin lines and the gray-scale and does not give any strange visual feeling. In addition, it is also described in the same reference that, in the full-color image forming method in which a full-color image is formed by overlaying on a transfer material toner images of each of at least four (4) colors of cyan, magenta, yellow and black, the reproducibility of thin lines and disorder of the image on the transfer material are improved, thickness of the image is reduced and a very high-quality image can be formed by employing toners satisfying the above requirements as the toners of the four colors used.
In Japanese Patent Application Laid-open (kokai) No. 2000-98657, the requirements for a carrier such as particle diameters, resistance that may be combined with the above toners are disclosed.
However, toners with small particle diameters will have stronger van der Waals force as the particle diameter becomes smaller, so that such toners will have a stronger force for adhering to carrier particles in comparison with the conventional toners. Therefore, when images are tried to be developed with the toners of small particle diameters by the conventionally known two-component magnetic brush method that is described in the above Japanese Patent Application Laid-open (kokai) Nos. 2000-81722 and 2000-305361, a satisfactory developing performance can not be obtained. Resulting visual images will suffer from, for example, decrease in image density and thinning of horizontal lines. When, for example, the linear velocity of the developing roller is increased extremely to secure the developing property, such phenomena as adhesion of the carrier to an image forming body comprising a photosensitive body (beads carry over) and scattering around of the carrier (carrier scattering) will occur.
Because of the reasons described above, it is necessary in practice to take some measures for improving the developing property when the toners with small particle diameters are used.
As a result of extensive discussions of formation of visual images using the toners with small particle diameters based on the situation described above, aiming at the improvement of the developing property, the inventors found that, in the conventional developing method utilizing an electric field, i.e., a method in which development is conducted by liberation of the toner particles electrostatically from the carrier by mainly the force of an electric field formed between the rotary sleeve for delivering the developer and the photosensitive body, it is difficult to liberate the toner particles from the carrier effectively when the toners with small particle diameters having large van der Waals force acting as the non-electrostatic adhering force are used, but the developing property can be improved because the toners with small particle diameters can be liberated from the carrier effectively when the two-component developer is strongly agitated under specific conditions.
In order to allow the developer to be agitated strongly, the approach of increasing the linear velocity of the rotary sleeve is commonly effective. However, in this approach, carrier adhesion and carrier scattering occur as a result of increased centrifugal force at the rotary sleeve and, thus, it is not an advantageous approach.
In addition, in order to have the stronger agitation of the developer, it has been found that lowering the height of the tip of the magnetic brush by reducing the amount of the developer on the rotary sleeve for preventing the developer from being packed in the developing area is effective in practice. That is, according to this approach, the agitation of the developer in the developing area can be made stronger by developing on magnetic poles with the two-component developer on the rotary sleeve while keeping the magnetic brush slightly in contact with the photosensitive body with the result that the toner with small particle diameters becomes easy to be liberated from the carrier and the developing efficiency is consequently improved.
The present invention has been made in view of the above findings. It is therefore the object of the present invention to provide an image forming apparatus which ensures acquisition of excellent developing properties using toner with small particle diameters and, thus, can form high-quality visual images.
According to a first aspect of the present invention there is provided an image forming apparatus which comprises an image forming body, and a developing device for developing a latent image formed on the image forming body with a two-component developer consisting of a toner and a carrier, the developing device including a rotary sleeve positioned facing the image forming body, for delivering the two-component developer and, a magnet system positioned inside the rotary sleeve, for forming a plurality of magnetic poles to form a magnetic brush made of the two-component developer on the surface of the rotary sleeve, and a developer layer regulating member positioned facing the surface of the rotary sleeve, for regulating the amount of the two-component developer delivered by the rotary sleeve, wherein where dt (xcexcm) represents the volume mean particle diameter of the toner in the two-component developer, xcfx81t (g/cm3) represents the density of the toner, dc (xcexcm) represents the volume mean particle diameter of the carrier, xcfx81c (g/cm3) represents the density of the carrier, and Rw represents the weight ratio of the toner and the carrier (the ratio of the weight of the toner to the weight of the carrier), Conditions 1, 2 and 3 are satisfied,
Condition 1: the volume mean particle diameter dt of the toner is within the range of 3xcx9c5 xcexcm,
Condition 2: the volume mean particle diameter dc falls within the range of 5 dtxcx9c10 dt, and
Condition 3: the weight ratio Rw of the toner and the carrier is within the range of 1.6 (dt/dc)xc3x97(xcfx81t/xcfx81c)xcx9c2.4 (dt/dc)xc3x97(xcfx81t/xcfx81c),
and wherein among a plurality of the magnetic poles included in the magnet system, a main magnetic pole, forming the strongest magnetic field on the surface of the rotary sleeve, is positioned in proximity to the position where the rotary sleeve and the image forming body come closest to each other, and wherein where H (mm) represents the free tip height of the magnetic brush formed at the position of the main magnetic pole, and D (mm) represents the closest distance between the rotary sleeve and the image forming body, Condition 4 is satisfied, Condition 4: the closest distance D is within the range of 0.5 Hxcx9c0.8 H.
In the above-described image forming apparatus, it is preferable that the following Condition 3A is satisfied for the weight ratio Rw of the toner and the carrier in the two-component developer;
Condition 3A: the weight ratio Rw of the toner and the carrier is within the range of 1.8 (dt/dc)xc3x97(xcfx81t/xcfx81c)xcx9c2.2 (dt/dc)xc3x97(xcfx81t/xcfx81c).
According to a second aspect of the present invention there is provided an image forming apparatus which comprises an image forming body, and a developing device for developing a latent image formed on the image forming body with a two-component developer consisting of a toner and a carrier, the developing device including a rotary sleeve positioned facing the image forming body, for delivering the two-component developer and, a magnet system positioned inside the rotary sleeve, for forming a plurality of magnetic poles to form a magnetic brush made of the two-component developer on the surface of the rotary sleeve, and a developer layer regulating member positioned facing the surface of the rotary sleeve, for regulating the amount of the two-component developer delivered by the rotary sleeve, wherein where dt (xcexcm) represents the volume mean particle diameter of the toner in the two-component developer, W (mg/cm2) represents the delivered amount per unit area of the two-component developer delivered by the rotary sleeve, Tc (weight percent) represents the toner concentration in the two-component developer, and Rv represents the ratio of the moving velocity of the rotary sleeve to the moving velocity of the image forming body, conditions;
Condition 1: the volume mean particle diameter dt of the toner falls within the range of 3xcx9c5 xcexcm,
Condition 5: the delivered amount W of the two-component developer falls within the range of 10xcx9c50 mg/cm2, and
Condition 6: the actual supplied amount of the toner represented by an expression, (Wxc3x97Tcxc3x97Rv)/100 falls within the range of 2xcx9c10 mg/cm2, are satisfied;
and wherein among a plurality of magnetic poles included in the magnet system, a main magnetic pole forming the strongest magnetic field on the surface of the rotary sleeve is positioned in proximity to the position where the rotary sleeve and the image forming body come closest to each other, and wherein where H (mm) represents the free tip height of the magnetic brush formed at the position of the main magnetic pole, and D (mm) represents the closest distance between the rotary sleeve and the image forming body, a condition,
Condition 4: the closest distance D falls within the range of 0.5 Hxcx9c0.8 H is satisfied.
In the above-described image forming apparatus of the invention, it is preferable that, where dt (xcexcm) represents the volume mean particle diameter of the toner in the two-component developer, xcfx81t (g/cm3) represents the density of the toner, dc (xcexcm) represents the volume mean particle diameter of the carrier, xcfx81c (g/cm3) represents the density of the carrier, and Rw represents the weight ratio of the toner and the carrier (the ratio of the weight of the toner to the weight of the carrier), conditions;
Condition 2: the volume mean particle diameter dc of the carrier is within the range of 5 dt-10 dt, and Condition 3 is safisfied,
Condition 3: the weight ratio Rw of the toner and the carrier falls within the range of 1.6 (dt/dc)xc3x97(xcfx81t/xcfx81c)xcx9c2.4 (dt/dc)xc3x97(xcfx81t/xcfx81c).
Furthermore, it is preferable that the following Condition 6A is satisfied,
Condition 6A: the actual supplied amount of the toner represented by an expression, (Wxc3x97Tcxc3x97Rv)/100 falls within the range of 4xcx9c8 mg/cm2.
In the above image forming apparatus, it is preferable that, in the developing device, a bias voltage consisting of a DC voltage superimposed with a AC voltage is applied to the rotary sleeve.
Furthermore, it is preferable that, in the developing area where the rotary sleeve faces the image forming body and the image forming device move in the same direction and that the main magnetic pole is the first magnetic pole downstream in the direction of movement of the rotary sleeve from the developer layer regulating member.
Additionally, in the developing device, the main magnetic pole is positioned upstream in the direction of movement of the rotary sleeve from the position where the rotary sleeve and the image forming body comes closest to each other.
The image forming apparatus of the present invention comprises a plurality of image forming bodies, each forming toner images of colors of yellow, magenta, cyan and black, respectively, and an intermediate transferring body on which each of the toner images formed on the plurality of image forming bodies is transferred and superimposed one after another and, constitutes an image forming apparatus forming colored images.
According to the above-mentioned image forming apparatus, development on the magnetic poles is conducted with the main magnetic pole formed by providing the magnetic poles in a specific arrangement using the two-component developer comprising the toner with small particle diameters satisfying Condition 1 and the carrier with small particle diameters satisfying Condition 2 contained at a ratio satisfying Condition 3, and a latent image on a photosensitive material drum is developed in the situation of a slight contact in which only the tip end of the magnetic brush B is in contact with the photosensitive material drum since the height of the free tip of the magnetic brush is in a specific situation satisfying Condition 4. Therefore, a visual image having a high image quality equal to or better than that of, for example, offset printing can be easily formed.
Furthermore, since the actual delivered amount of the toner delivered actually to the developing area P is secured by satisfying Condition 5 and Condition 6, the lowering of the image density is reliably prevented and, therefore, a high-quality visual image can be reliably formed.